On 12/08/2010 05:15 PM, Pauli Nieminen wrote:
On 08/12/10 16:55 +0100, ext Alex Deucher wrote:
One other thing that might be worth adding to DRI2 is a way for the
driver to access the swap interval. If we could, then the driver
could dynamically disable things like vline waits for buffer blits or
do non-vsynced pageflipping more easily if the swap interval was 0.
Actually I tough DRI2 was already telling SwapInteval with
*swap_target = pPriv->last_swap_target + pPriv->swap_interval;
If swap_target doesn't advance then driver would know to flip as fast as
possible.
But then I noticed a bug.
First one is that we always blit with swapinterval is zero when it should be
possible to flip (even tear free).
Let it tear :). I think swap_interval == 0 should cause immediate swaps
with tearing. All OpenGL implementations i know (Windows, MacOS classic,
MacOS/X, Linux with proprietary drivers, old SGI's) interpret a
swapinterval of zero as "swap as soon as rendering is finished,
immediately, don't sync to scanout cycle" and i think that makes the
most sense. It's good for benchmarking how fast your system can go if
not throttled by the monitor (useful), for crazy gamers that trade
visual quality for fps and for special applications that need to
sometimes control swap timing themselves (like my toolkit).
Code from DRI2ScheduleSwap:
/* Old DDX or no swap interval, just blit */
if (!ds->ScheduleSwap || !pPriv->swap_interval) {
Not a "bug", just a half-done way of achieving the expected
swap_interval zero ;-) [ok, maybe that is a bug].
It schedules an immediate copy-swap via blitting. Unfortunately the ddx
doesn't know about the swap_interval, so it still synchronizes the
execution of the blit to vsync via vline waits. That's tear-free, but it
depends on the location and size of the drawable and the current
position of the scanout if this will cause an immediate swap (if scanout
is outside the drawables area) or a vsync'ed swap. It's a bit undefined
behaviour for non-fullscreen drawables and it effectively enforces a
minimum swap interval of 1 for fullscreen drawables, which is not what
we want.
Both the intel ddx and (soon) the ati ddx have a xorg.conf parameter
"SwapBuffersWait" to disable vsync completely, then you'd get copy-swaps
as fast as possible for swap_interval zero, but for a non-zero
swap_interval you'd have some chance of tearing, so this is alos just a
band-aid.
If we wanted this for page-flipping we'd need a shortcut similar to this
one which would bypass the vblank scheduling and call the ddx pageflip
routine directly, plus an extension to the kernel's pageflip ioctl() to
allow non-vsynced flips.
And we need a new interface to tell the ddx that a swap should be
non-vsync'ed.
-mario
_______________________________________________
[email protected]: X.Org development
Archives: http://lists.x.org/archives/xorg-devel
Info: http://lists.x.org/mailman/listinfo/xorg-devel
